drm/panfrost: Remove opp table when unloading

The devfreq opp table needs to be removed when unloading the driver to
free the memory associated with it.

Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190816093107.30518-3-steven.price@arm.com